Impedance has been measured by applying an excitation signal to a device with an unknown impedance and measuring characteristics of a signal returned from the device. Specifically, a signal analyzer may “sweep” the frequencies of the return signal to calculate a magnitude and phase of the impedance. The excitation signal may be generated by converting a digital sine wave signal to an analog signal at a digital to analog converter (DAC). The analog signal may then be filtered to remove aliasing images. The filtered analog signal may then be passed through the device and the return signal may be filtered again to remove high frequency components. An analog to digital converter (ADC) may sampled the filtered return signal to generate a digital code. The digital code may be analyzed using Fourier analysis to identify individual spectral components within the digital code and calculate the impedance.
FIG. 1 shows an example of an existing impedance measuring circuit 100. In the past, the DAC 102 and ADC 105 have both used a same or equivalent clock signal 111, in that the clock signal of the DAC 102 and ADC 105 had a constant relative phase between them. This arrangement required the use of a reconstruction filter 103 and/or an anti-aliasing filter 104 that sufficiently reduced the bandwidth of the analog signal to prevent aliasing between the sampled signal and its images. Since both the DAC 102 and ADC 105 used the same clock, the filters 103 and 104 must be configured to bandlimit the analog signal in order to prevent aliasing, which may cause higher frequency waves to be represented as lower frequency waves.
If these filters 103 and 104 did not sufficiently limit the bandwidth of the analog signal, the overlap between the edges of the images will improperly increase the amplitude of the signal in the overlapping region. This will cause signal processing errors for those frequencies of interest in the overlapping region. Thus, the filters 103 and 104 must bandlimit the analog signal to prevent aliasing in those regions associated with possible frequencies of interest. This requirement may require filters 103 and 104 that are more expensive and in some case impractical for their intended use.
The inventors perceive a need to prevent increased signal processing errors for particular frequencies of interest in an aliasing region using a relaxed anti-aliasing filter.